﻿558 
  Prof. 
  J. 
  S. 
  Townsend 
  on 
  the 
  Conductivity 
  

  

  were 
  measured 
  between 
  two 
  parallel 
  plates 
  at 
  different 
  dis- 
  

   tances 
  apart. 
  The 
  light 
  fell 
  on 
  the 
  negative 
  electrode, 
  which 
  

   was 
  a 
  zinc 
  plate. 
  The 
  light 
  has 
  the 
  effect 
  of 
  setting 
  free 
  

   negative 
  ions 
  from 
  the 
  surface 
  of 
  the 
  metal 
  on 
  which 
  it 
  falls. 
  

   These 
  ions, 
  which 
  I 
  shall 
  call 
  the 
  z 
  ions, 
  as 
  they 
  move 
  through 
  

   the 
  gas 
  under 
  an 
  electric 
  force 
  produce 
  others 
  by 
  collisions 
  

   with 
  molecules. 
  The 
  increase 
  of 
  conductivity 
  obtained 
  by 
  

   separating 
  the 
  plates 
  is 
  thus 
  explained. 
  The 
  experiments 
  

   also 
  lead 
  to 
  the 
  conclusion 
  that 
  the 
  ions 
  generated 
  in 
  the 
  gas 
  

   have 
  exactly 
  the 
  same 
  properties 
  as 
  the 
  z 
  ions. 
  In 
  order 
  

   to 
  show 
  this, 
  the 
  conductivity 
  was 
  determined 
  for 
  different 
  

   distances 
  between 
  the 
  plates, 
  the 
  electric 
  force, 
  X, 
  and 
  the 
  

   pressure, 
  p, 
  of 
  the 
  gas 
  being 
  kept 
  constant. 
  The 
  number 
  of 
  

   ions, 
  a, 
  generated 
  by 
  one 
  of 
  the 
  z 
  ions 
  in 
  moving 
  through 
  a 
  

   centimetre 
  of 
  gas 
  is 
  a 
  function 
  of 
  X 
  and 
  p, 
  so 
  that 
  ex 
  

   remains 
  constant 
  in 
  the 
  experiments 
  with 
  different 
  distances 
  

   between 
  the 
  plates. 
  If 
  the 
  negative 
  ions 
  generated 
  in 
  the 
  

   gas 
  have 
  the 
  same 
  properties 
  as 
  the 
  z 
  ions, 
  the 
  conductivity 
  

   will 
  increase 
  by 
  the 
  factor 
  € 
  aX 
  when 
  the 
  distance 
  between 
  the 
  

   plates 
  is 
  increased 
  by 
  the 
  length 
  x. 
  The 
  total 
  number 
  of 
  ions 
  

   n 
  reaching 
  the 
  positive 
  plate 
  will 
  be 
  given 
  by 
  the 
  formula 
  

  

  n 
  = 
  n 
  6 
  ad 
  , 
  

  

  n 
  Q 
  being 
  the 
  number 
  of 
  ions 
  set 
  free 
  from 
  the 
  zinc 
  plate, 
  and 
  

   d 
  the 
  distance 
  between 
  the 
  plates. 
  It 
  was 
  found 
  that 
  this 
  

   formula 
  gives 
  the 
  connexion 
  between 
  the 
  values 
  of 
  n 
  and 
  d 
  

   over 
  a 
  large 
  range 
  of 
  forces 
  and 
  pressures. 
  

  

  2. 
  The 
  apparatus 
  used 
  in 
  these 
  experiments 
  is 
  shown 
  in 
  

   figures 
  1 
  and 
  2. 
  

  

  The 
  conductivity 
  took 
  place 
  between 
  the 
  two 
  parallel 
  

   plates 
  A 
  and 
  B, 
  figure 
  1. 
  The 
  plate 
  A 
  was 
  of 
  quartz 
  silvered 
  

   on 
  the 
  inside 
  so 
  as 
  to 
  obtain 
  a 
  conducting 
  surface. 
  The 
  

   silvering 
  was 
  ruled 
  with 
  a 
  series 
  of 
  fine 
  parallel 
  lines 
  to 
  

   allow 
  some 
  of 
  the 
  light 
  to 
  pass 
  through 
  the 
  plate 
  and 
  fall 
  on 
  

   the 
  surface 
  of 
  the 
  lower 
  plate 
  B, 
  which 
  was 
  of 
  zinc. 
  The 
  

   plate 
  A 
  rested 
  on 
  a 
  brass 
  cap 
  C, 
  and 
  the 
  junction 
  between 
  the 
  

   plate 
  and 
  the 
  brass 
  was 
  made 
  air-tight. 
  The 
  short 
  ebonite 
  

   tube 
  E 
  was 
  screwed 
  to 
  the 
  flat 
  part 
  of 
  the 
  cap 
  on 
  which 
  the 
  

   quartz 
  plate 
  rested, 
  and 
  a 
  metal 
  disk 
  F 
  was 
  fixed 
  to 
  the 
  lower 
  

   end 
  of 
  the 
  ebonite. 
  This 
  disk 
  was 
  thus 
  in 
  rigid 
  connexion 
  

   with 
  the 
  quartz 
  plate 
  A 
  and 
  parallel 
  to 
  it. 
  The 
  distance 
  

   between 
  the 
  plates 
  A 
  and 
  B 
  could 
  be 
  adjusted 
  by 
  means 
  of 
  

   the 
  screw 
  S, 
  which 
  worked 
  in 
  a 
  half-millimetre 
  thread 
  cut 
  

   in 
  the 
  plate 
  F. 
  The 
  wide 
  glass 
  tube 
  D 
  which 
  contained 
  the 
  

   apparatus 
  was 
  ground 
  flat 
  at 
  one 
  end, 
  and 
  the 
  brass 
  cap 
  was 
  

   cemented 
  to 
  it. 
  The 
  other 
  end 
  was 
  joined 
  to 
  a 
  narrower 
  

  

  